Optoelectronic devices

ABSTRACT

An optoelectronic device comprising a photoresponsive region located between first and second electrodes such that charge carriers can move between the photoresponsive region and the first and second electrodes, the photoresponsive region comprising: a stack of alternate first and second layers, each first layer comprising a first photoresponsive material and each second layer comprising a second photoresponsive material, the first and second photoresponsive materials having different electron affinities; wherein each pair of second layers on opposite sides of a first layer contact each other via first holes defined by said first layer between said pair of second layers, and each pair of first layers on opposite sides of a second layer contact each other via second holes defined by said second layer between said pair of first layers.

The present invention relates to optoelectronic devices which comprise aphotoresponsive region and in particular to devices intended to detectlight such as photodetectors and to devices intended to provide a sourceof electrical energy from electromagnetic radiation such as solar cells.

A photodetector or a solar cell typically comprises a photoresponsivematerial located between two electrodes, wherein the electric potentialbetween the two electrodes changes when the photoresponsive material isexposed to light.

U.S. Pat. No. 5,670,791 discloses a polymeric photodetector comprising alayer of a blend of poly(2-methoxy-5(2′-ethyl)hexyloxyphenylenevinylene)(“MEH-PPV”) and poly(cyanophenylene-vinylene) (“CN-PPV”) located betweenan indium-tin oxide (ITO) electrode and an aluminum electrode.

Relatively good device efficiencies have been achieved with thesedevices, but there is a demand for photodetectors having even higherdevice efficiencies.

It is therefore an aim of the present invention to provide aphotoresponsive device having an improved device efficiency.

According to a first aspect of the present invention, there is providedan optoelectronic device comprising a photoresponsive region locatedbetween first and second electrodes such that charge carriers can movebetween the photoresponsive region and the first and second electrodes,the photoresponsive region comprising: a stack of alternate first andsecond layers, each first layer comprising a first photoresponsivematerial and each second layer comprising a second photoresponsivematerial, the first and second photoresponsive materials havingdifferent electron affinities; wherein each pair of second layers onopposite sides of a first layer contact each other via first holesdefined by said first layer between said pair of second layers, and eachpair of first layers on opposite sides of a second layer contact eachother via second holes defined by said second layer between said pair offirst layers.

According to one embodiment, the first electrode is adjacent one of saidfirst layers and the second electrode is adjacent one of said secondlayers.

The first holes defined by each first layer between each pair of secondlayers are preferably arranged in an ordered array, and the second holesdefined by each second layer between each pair of first layers are alsopreferably arranged in an ordered array.

Preferably, both the first and second photoresponsive materials aresemi-conductive conjugated polymers.

According to one embodiment, each first layer between a pair of secondlayers comprises a blend of the first and second photoresponsivematerials, the proportion of first photoresponsive material being higherthan the proportion of second photoresponsive material; and each secondlayer between a pair of first layers also comprises a blend of the firstand second photoresponsive materials, the proportion of secondphotoresponsive material being higher than the proportion of the firstphotoresponsive material.

According to a second aspect of the present invention, there is provideda method of producing an optoelectronic device comprising aphotoresponsive region located between first and second electrodes suchthat charge carriers can move between the photoresponsive region and thefirst and second electrodes, wherein said photoresponsive region isformed by a process comprising the steps of: depositing an array offirst regions on a substrate comprising the first electrode, wherein thearray of first regions define holes therebetween exposing portions ofthe underlying substrate; and depositing an array of second regions inthe holes defined between the first regions such that the second regionspartially overlap the first regions, and define holes therebetweenexposing the portions of the underlying first regions; wherein the firstregions comprise a first photoresponsive material and the second regionscomprise a second photoresponsive material, the first photoresponsivematerial and second photoresponsive material having different electronaffinities.

According to a preferred embodiment, the array of first regions definean array of distinct holes, and the array of second regions define anarray of distinct holes.

According to a preferred embodiment, the arrays of first and secondregions are deposited by an ink-jet printing technique.

According to a third aspect of the present invention, there is providedan optoelectronic device comprising a photoresponsive region locatedbetween first and second electrodes such that charge carriers can movebetween the photoresponsive region and the electrodes; thephotoresponsive region comprising at least first and second blend layerseach comprising a blend of a first photoresponsive material and a secondphotoresponsive material having differing electron affinities, whereinthe first and second blend layers comprise different proportions of thefirst and second photoresponsive materials.

Preferably, at least one of the photoresponsive materials is asemi-conductive conjugated polymer, and further preferably, both of thephotoresponsive materials are semi-conductive conjugated polymers.

According to a preferred embodiment, the photoresponsive regioncomprises a stack of alternating first and second blend layers.

The photoresponsive region preferably further comprises a layerconsisting essentially of the first photoresponsive material adjacentthe first electrode, and/or a layer consisting essentially of the secondphotoresponsive material adjacent the second electrode. These layersfacilitate the transport of the respective charge carriers into thefirst and second electrodes.

Embodiments of the invention shall be described hereunder, by way ofexample, only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic partial cross-sectional view of an optoelectronicdevice according to a first embodiment of the present invention;

FIGS. 2( a) and 2(b) show the relationship between adjacent layers ofthe device shown in FIG. 1; and

FIG. 3 is a schematic view of an optoelectronic device according to asecond embodiment of the present invention.

With reference to FIGS. 1, 2(a) and 2(b), a method for producing anoptoelectronic device according to a first embodiment of the presentinvention is described below. A glass base 2 is coated with a layer ofindium tin oxide (ITO) to provide an anode layer 4. Next, a continuouslayer of MEH-PPV 6 is deposited on the surface of the ITO layer oppositethe glass substrate by a standard deposition technique such asblade-coating, spin-coating or an ink-jet printing technique. TheMEH-PPV layer 6 has a thickness in the range of 5 to 100 nm, preferably5 to 20 nm. Next, an ink-jet printing technique is used to deposit anarray droplets of a solution of CN-PPV in an appropriate solvent on mesurface of the MEH-PPV layer opposite the ITO layer.

The volume of each droplet and the spacing of the droplets arecontrolled such that after drying to evaporate She solvent, thereremains a layer of CN-DPV spots 8 left on the MEH-PPV layer whichcontact each other (as shown in FIG. 1 a) or overlap each other (asshown in FIG. 2 b) at their circumference but also define distinct holes10 therebetween which expose portions of the underlying MEH-PPV layer 6.The outline of the CN-PPV spots 8 are shown by broken lines in FIGS. 2(a) and 2(b). Next, an ink-jet printing technique is used to deposit anarray of droplets of a solution of MEH-PPV in an appropriate solvent onthe layer of CN-PPV spots 8 such that after drying to evaporate solvent,there remains a layer of MEH-PPV spots 12 which fill the roles 10defined by the underlying array of CN-PPV spots 8 and partially coverthe underlying CN-PPV spots 8 and define an array of distinct holes 14therebetween which expose portions of the underlying CN-PPV spots 8. TheMEH-PPV spots 12 (as shown by full lines in FIGS. 2( a) and 2(b)) alsocontact wait each other (as shown in FIG. 2( a)) or overlap with eachother (as shown in FIG. 2( b)) at their circumference.

Examples of suitable solvents for the solutions of MEH-PPV and CN-PPVinclude xylene, toluene and THF. It is preferable to use the samesolvent for both MEH-PPV and CN-PPV with a view to achieving someintermixing. Each layer should be dried before deposition of the nextlayer.

This procedure is repeated to produce an alternating stack of layers ofCN-PPV and MEH-PPV spots of the desired thickness, with a layer ofMEH-PPV spots as the top layer. The stack preferably has a thicknesscorresponding to an optical density of between 1 and 3. The thickness ofthe stack is preferably in the range of 100 to 400 nm.

Next, a continuous layer of CN-PPV 16 is deposited on the top layer ofMEH-PPV spots 12 by a standard technique such as blade-coating,spin-coating or an ink-jet printing technique, and a cathode 18 isformed on the surface of the continuous layer of CN-PPV opposite the toplayer of MEH-PPV spots. The cathode 18 preferably comprises a layer ofaluminum or has a bi-layer structure comprising an underlying thin layerof a metal having a low work function such as calcium and an overlyingthicker layer of aluminum.

For the sake of simplicity, FIG. 1 shows a schematic partialcross-sectional view of a device having only two layers of MEH-PPV spots12 and two layers of CN-PPV spots 8 wherein the layers of spots have thepattern generally shown in FIG. 2( a). FIG. 1 is a schematic partialcross-sectional view taken through line A—A in FIG. 2( a).

In an alternative modification, the MEH-PPV spots 8 could be replaced byMEH-PPV/CN-PPV spots comprising a blend of a major proportion of MEH-PPVand a minor proportion of CN-PPV. Similarly, the CN-PPV spots 12 couldbe replaced by CN-PPV/MEH-PPV spots comprising a blend of a majorproportion of CN-PPV and a minor proportion of MEH-PPV.

In the product device, the thickness of the spots is in the range of 5nm to 1000 nm, preferably 5 nm to 20 nm, and the diameter of the spotsis in the range of 0.5 to 1000 microns, preferably 5 to 50 microns.

The close proximity of photoresponsive materials having differingelectron affinities ensures efficient charge separation when an excitonis formed within the photoresponsive region upon exposure of thephotoresponsive region to light. The thickness of the spots ispreferably made as small as possible to minimise the lateral diffusionlength i.e. the distance that a charge carrier needs to travel beforecollection in an area of high or low electron affinity, as the case maybe, to thereby maximise the device efficiency.

The use of an ink-jet printing technique to deposit the ordered array ofdroplets means that droplets of very small size can be depositedresulting in an ordered array of regions of different photoresponsivematerials having a very small size, thereby making it possible toproduce a device having a very good device efficiency.

Furthermore, the feature that the array of CN-PPV and MEH-PPV spots isordered ensures that the lateral diffusion length is uniformly smallwherever the exciton is formed within the photoresponsive region,thereby ensuring that the device exhibits a uniform sensitivity acrossthe whole area of the photoresponsive region.

As described above, the alternate two-dimensional ordered arrays ofMEH-PPV spots and CN-PPV spots are staggered, resulting in paths fromeach MEH-PPV spot to the anode which only pass through MEH-PPV, andpaths from each CN-PPV spot to the cathode which only pass throughCN-PPV. The provision of such paths increases the efficiency ofcollection of the respective charge carriers by the anode and cathode,respectively, thereby further maximising the efficiency of the device.

Next, with reference to FIG. 3, a second embodiment of the presentinvention will be described.

A layer of MEH-PPV 34 is deposited on a glass base 30 provided with ananode layer 32. The anode layer 32 is typically a layer of ITO. A layerof a CN-PPV/MEH-PPV blend 36 comprising a major proportion of CN-PPV anda minor proportion of MEH-PPV is deposited on the MEH-PPV layer 34. Theproportional ratio of MEH-PPV to CN-PPV is preferable in the range of5:95 to 40:60. In this embodiment, the ratio is 20:80. Next, a layer 38of a MEH-PPV/CN-PPV blend comprising a major proportion of MEH-PPV and aminor proportion of CN-PPV is deposited on the underlying CN-PPV/MEH-PPVlayer 36. The proportional ratio of MEH-PPV to CN-PPV is preferably inthe range of 95:5 to 60:40. In this embodiment, the ratio is 80:20. Thisdeposition of alternating CN-PPV/MEH-PPV and MEH-PPV/CN-PPV blend layersis repeated until a stack of the desired number of layers is achievedwith the top layer being a MEH-PPV/CN-PPV blend layer. Next, a layerconsisting of CN-PPV is formed on the top MEH-PPV/CN-PPV blend layer,followed by the formation of a cathode 42 on the surface of the CN-PPVlayer 40. The cathode typically comprises one or more metal layers. In apreferred embodiment, it comprises a thin layer of a metal having a lowwork function such as calcium and an overlying layer of aluminum.

In the final device, the MEH-PPV layer and the CN-PPV layer adjacent theanode and cathode, respectively have a thickness in the range of 5 to3000 Å, preferably 50 to 150 Å, and the MEH-PPV/CN-PPV andCN-PPV/MEH-PPV blend layers have a thickness in the range of 5 to 3000Å, preferably 50 to 150 Å.

The stack preferably has a total thickness sufficient to absorb about atleast 50% of light passing through the stack having a wavelength in therange of 350 to 800 nm. If, for example, the device is then providedwith a high reflectivity cathode, about 75% or greater of light incidenton the stack will be absorbed as incident light which is not absorbed bythe stack on the first pass is reflected back through the stack.Preferably, the stack has a total thickness sufficient to absorb about90% of the light.

Any polymer deposition technique such as spin-coating or blade-coatingcan be used to deposit the polymer and polymer blend layers. An ink-jetprinting technique is preferred because it can be used to produce layersof very small thicknesses.

In the embodiments described above, MEH-PPV and CN-PPV have been used asphotoresponsive materials having different electron affinities. However,it will be understood to the person skilled in the art that the presentinvention is not limited to the use of these specific materials but isgenerally applicable to any appropriate combination of photoresponsivematerials having different electron affinities.

1. An optoelectronic device comprising a photoresponsive region locatedbetween first and second electrodes such that charge carriers can movebetween the photoresponsive region and the first and second electrodes,the photoresponsive region comprising: first and second layersalternately stacked in a direction extending between the first andsecond electrodes, each first layer comprising a first photoresponsivematerial and each second layer comprising a second photoresponsivematerial, the first and second photoresponsive materials havingdifferent electron affinities; wherein each pair of second layers onopposite sides of a first layer contact each other via first holesdefined by said first layer between said pair of second layers, and eachpair of first layers on opposite sides of a second layer contact eachother via second holes defined by said second layer between said pair offirst layers.
 2. An optoelectronic device according to claim 1 whereinthe first electrode is adjacent one of said first layers and the secondelectrode is adjacent one of said second layers.
 3. An optoelectronicdevice according to claim 1 wherein the first holes defined by eachfirst layer between each pair of second layers are arranged in anordered array.
 4. An optoelectronic device according to claim 1 whereinthe second holes defined by each second layer between each pair of firstlayers are arranged in an ordered array.
 5. An optoelectronic deviceaccording to claim 1 wherein each first layer between a pair of secondlayers comprises an ordered array of interconnecting first regions whichdefine said first holes therebetween, and each second layer between apair of first layers comprises an ordered array of interconnectingsecond regions which define said second holes therebetween.
 6. Anoptoelectronic device according to claim 1, wherein each first layerbetween a pair of second layers comprises a blend of the first andsecond photoresponsive materials, the proportion of firstphotoresponsive material being higher than the proportion of secondphotoresponsive material; and each second layer between a pair of firstlayers also comprises a blend of the first and second photoresponsivematerials, the proportion of second photoresponsive material beinghigher than the proportion of the first photoresponsive material.
 7. Anoptoelectronic device according to claim 1 wherein the firstphotoresponsive material and second photoresponsive material aresemi-conducting conjugated polymers.
 8. A method of producing anoptoelectronic device comprising a photoresponsive region locatedbetween first and second electrodes such that charge carriers can movebetween the photoresponsive region and the first and second electrodes,wherein said photoresponsive region is formed by a process comprisingthe steps of: depositing an array of first regions on a substratecomprising the first electrode, wherein the array of first regionsdefine holes therebetween exposing portions of the underlying substrate;and depositing an array of second regions in the holes defined betweenthe first regions such that the second regions partially overlap thefirst regions and define holes therebetween exposing the portions of theunderlying first regions; wherein the first regions comprise a firstphotoresponsive material and the second regions comprise a secondphotoresponsive material, the first photoresponsive material and secondphotoresponsive materials having different electron affinities.
 9. Amethod according to claim 8 wherein the array of first regions define anarray of distinct holes.
 10. A method according to claim 8 wherein thesecond regions define an array of distinct holes.
 11. A method accordingto claim 8 wherein the process of forming the photoresponsive regionfurther comprises the step of forming an array of third regionscomprising the first photoresponsive material in the holes defined bythe array of second regions such that the third regions partiallyoverlap the second regions and define holes therebetween exposingportions of the underlying second regions.
 12. A method according toclaim 8 wherein the process of forming the photoresponsive regionfurther comprises the step of forming an array of fourth regionscomprising the second photoresponsive material in the holes defined bythe third regions such that the fourth regions partially overlap thethird regions and define holes therebetween exposing portions of theunderlying third regions.
 13. A method according to claim 8 wherein thesubstrate comprising the first electrode further comprises a continuouslayer of the second photoresponsive material, and the first regions areformed on the continuous layer of the second photoresponsive material.14. A method according to claim 13 wherein the photoresponsive regionfurther comprises a continuous layer of first photoresponsive materialadjacent the second electrode.
 15. An optoelectronic device comprising aphotoresponsive region located between first and second electrodes suchthat charge carriers can move between the photoresponsive region and theelectrodes; the photoresponsive region comprising at least first andsecond blend layers each comprising a blend of a first photoresponsivematerial formed from a semi-conductive conjugated polymer having a firstelectron affinity and a second photoresponsive material having adiffering electron affinity, wherein the first and second blend layerscomprise different proportions of the first and second photoresponsivematerials.
 16. An optoelectronic device comprising a photoresponsiveregion located between first and second electrodes such that chargecarriers can move between the photoresponsive region and the electrodes;the photoresponsive region comprising a stack of alternating first andsecond blend layers each comprising a blend of a first photoresponsivematerial and a second photoresponsive material having differing electronaffinities, with at least one blend layer being formed from asemi-conductive conjugated polymer, wherein the first and second blendlayers comprise different proportions of the first and secondphotoresponsive materials.
 17. An optoelectronic device comprising aphotoresponsive region located between first and second electrodes suchthat charge carriers can move between the photoresponsive region and theelectrodes; the photoresponsive region comprising at least first andsecond blend layers each comprising a blend of a first photoresponsivematerial formed from a semi-conductive conjugated polymer having a firstelectron affinity and a second photoresponsive material having adiffering electron affinity, wherein the first and second blend layerscomprise different proportions of the first and second photoresponsivematerials and a layer consisting essentially of the firstphotoresponsive material is disposed adjacent the first electrode. 18.An optoelectronic device comprising a photoresponsive region locatedbetween first and second electrodes such that charge carriers can movebetween the photoresponsive region and the electrodes; thephotoresponsive region comprising at least first and second blend layerseach comprising a blend of a first photoresponsive material formed froma semi-conductive conjugated polymer having a first electron affinityand a second photoresponsive material having a differing electronaffinity, wherein the first and second blend layers comprise differentproportions of the first and second photoresponsive materials and alayer consisting essentially of the second photoresponsive material isdisposed adjacent the second electrode.